How to show image after import
In 2.79, you would have had to switch the view to Textured mode to see the image on the plane in the viewport. The equivalent in 2.8 would be to switch to LookDev.

The option is there, now, to see a reference image in Solid or Wireframe mode as well, by creating an Image Empty object, which is altogether more flexible.

You can create it manually, with ShiftA > Empty > Image, and adjusting the settings in its Data tab, or simply by dragging an image from your filesystem into a 3D View.